1. Field of the Invention
This invention relates to an optical system for an endoscope having a wide angle view, and more particularly to a wide angle optical system for an endoscope which is capable of varying its focal length for varying the magnification of the image viewed.
2. Description of the Prior Art
Recently, there has been a great demand for an endoscope for viewing the interior of a human body which has a large angle of view and has a thin flexible tube to be inserted into the body. A thinner tube is highly desirable since it decreases the possibility of pain when inserted into the body.
However, in order to make the angle of view wide and the thickness of the flexible tube or the optical fiber bundle of the endoscope small, the size of the image viewed would normally be greatly reduced. A reduced image would cause the eyes of the operator to become tired and lower the diagnostic efficiency and accuracy of the endoscope. Further, the smaller the image becomes, the more difficult become the operation of forceps and the cannulation of the duodenum. It becomes very difficult for doctors who are accustomed to use forceps with a conventional angle of view and image size to use them with the large angle of view.
For the above reasons, it is desirable to have means to enlarge the angle of view of the eyepiece when desired. There are three methods for enlarging the angle of view or changing the magnification of the eyepiece of an endoscope, as follows.
The first method is to mount an adaptor consisting of an optical system having the proper magnification on the eyepiece to change the effective magnification of the eyepiece. The second method is to incorporate a magnification changing lens into the lens system of the eyepiece. The third method is to make a part or the whole of the eyepiece movable to change the magnification.
The first method, however, is disadvantageous in that it requires an additional operation to mount the adapter on or remove the same from the eyepiece. The second method is disadvantageous in that inevitably the size and weight of the eyepiece portion of the endoscope becomes larger. Further, the operation of this endoscope becomes more complex and the brightness of the images viewed through the lens markedly changes when a magnification changing lens is incorporated.
The third method, however, is advantageous in that the magnification can easily be changed with a simple operation, not requiring the troublesome operations of mounting and removing the lens. Further, the lens group can be made so that it has a compact size and light weight. In addition, diopter adjustments can easily be made by simply moving a part of the eyepiece lens system. Furthermore, while the external shape of the eyepiece portion may not be changed in the second method, any accessory, like a camera or a visualizing scope, can easily be mounted on the eyepiece in this third method.